It is known in the art to mix mica into a polypropylene resin. For example, Japanese Patent Publication No. 18615/74 discloses a method of using mica having an aspect ratio (diameter/thickness) of 30 or more. Furthermore, Japanese Patent Publication Nos. 17544/83, 17545/83 and 18946/83, and Japanese Patent Application (OPI) No. 149452/81 disclose methods of using a modified polyolefin and/or amino silane compound, isocyanate compound, epoxy compound or the like together with mica (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). These methods impart an improved rigidity and tensile strength but an extremely insufficient impact strength to resin compositions.
A number of methods have been proposed to improve impact strength. For example, Japanese Patent Application (OPI) No. 162743/79 discloses a method of restricting the aspect ratio and amount of mica to be added within a specified range. Japanese Patent Application (OPI) No. 84837/83 discloses a method of restricting the amount, aspect ratio and average flake diameter of mica to be added within a specified range. Furthermore, Japanese Patent Application (OPI) No. 108241/83 discloses a method of using a polypropylene having a specified composition. However, none of these methods provide sufficient improvements in the balance between rigidity and impact strength, especially in falling weight impact strength. Therefore, these methods find difficulties in improving thermal resistance and scratch resistance of formed products while maintaining high impact strength.
The inventors have made various studies to overcome the above problems in the prior art. As a result, the inventors studied to the particle diameter and particle size distribution of mica to be added. Particularly, the inventors found that a resin composition excellent in balance between rigidity and thermal resistance, especially in falling weight impact strength, and thermal resistance which provides an excellent appearance (less scratched and warped) when formed in products can be obtained by mixing mica having an average particle diameter and particle size distribution specified within a finely divided particle size range or rubber-like material into a crystalline propylene-ethylene copolymer resin. Thus, this invention has been achieved.